This invention relates to a process for the preparation of alicyclic isocyanates starting from the corresponding aromatic isocyanate.
Organic isocyanates are used widely as a material of polyurethane for foamed mold goods, elastomer coatings, adhesives and others and an intermediate of agricultural chemicals, medicines and the like. Among the organic isocyanates an aromatic diisocyanate is important for the polyurethane material and used in large quantities. However, since polyurethanes prepared from the aromatic isocyanates have a serious disadvantage of yellowing, they are unsuitable for the use in coatings. In this field a non-yellowing aliphatic diisocyanate or alicyclic isocyanate are solely used.
These organic isocyanates are produced in a commercial scale by reaction of the corresponding amine with phosgene. With the noticeable development of the polyurethan industry tolylene diisocyanate (hereinafter referred to as TDI) and diphenylmethanediisocyanate (hereinafter referred to as MDI) have been produced in large quantities and inexpensively and accordingly, the intermediates, i.e. tolylenediamine (hereinafter referred to as TDA) and diaminodiphenylmethane (hereinafter referred to as MDA) have become available inexpensively and easily. Thus, starting from these aromatic diamines, the corresponding alicyclic amine has been prepared by hydrogenation reaction, from which an alicyclic isocyanate has been then prepared by the reaction with phosgene as disclosed in, for example, U.S. Pat. No. 3,856,862, No. 3,591,635, No. 2,822,373 and G.B. Pat. No. 1,127,338.
However, owing to the two steps of the hydrogenation of the aromatic nucleus and the reaction with phosgene, the preparation of alicyclic isocyanates is expensive as compared with the preparation of TDI and MDI. In this way the demand for these alicyclic isocyanates is limited because of the high preparation cost.